Field of the Invention
The present invention relates to masks for photolithographic processes and to methods of fabricating such masks. The present invention relates, in particular, to photolithography masks for the patterning of radiation-sensitive resist layers on semiconductor substrates for the fabrication of large scale integrated semiconductor components, and to methods for fabricating these masks.
In the course of the ever decreasing structure dimensions for the production of large scale integrated semiconductor components, a dimensionally accurate photolithographic transfer of mask structures to radiation-sensitive resist layers is becoming more and more important. In the meantime, semiconductor components are fabricated with structure line widths of 180 nm or less for commercial use in large volumes, so that the requirements made of the patterning process steps must satisfy very high standards. In this case, the photolithographic transfer of mask structures to radiation-sensitive resist layers is one of the outstanding techniques for patterning layers on semiconductor components.
In this case, the photolithographic transfer of mask structures to a radiation-sensitive resist layer is effected in a plurality of steps. The alignment of the mask above the substrate covered with the radiation-sensitive resist layer is followed by the exposure of the radiation-sensitive resist layer through the mask for marking the resist layer material to be removed (or to be left). In this case, the exposure of the radiation-sensitive resist layer can be effected in the silhouette method, the mask bearing on the resist layer (contact exposure) or being applied closely above the resist layer (proximity exposure). For very high resolution patterning, on the other hand, the exposure is carried out by means of a so-called projection exposure. In this case, the light that has passed through the mask is focused in a projection objective onto the resist layer, the projection objective imaging the maximum possible number of higher orders of diffraction produced by the mask structure. This imaging method makes it possible to image a minimum transferable structure line width R ofR=k·λ/NA  (1)from the mask onto the resist layer. In this case, λ is the wavelength with which irradiation or exposure is effected, NA is the numerical aperture and k is an empirical constant whose value nowadays is about 0.4. The theoretical limit for a line grating of a period 2R is k=0.25.
If the radiation-sensitive resist layer is a positive resist layer, then the exposure brings about at the exposed locations a chemical alteration of the resist layer material, which can be flushed out from the resist layer during development. By contrast, if the radiation-sensitive resist layer is a negative resist layer, then the non-exposed material is flushed out during development. In order to obtain the same structure as in the case of the positive resist, the mask must be patterned essentially complementarily with respect to the mask for the positive resist.
The exposure is followed by the development of the resist layer by spraying or, for example, pouring on developer liquid which selectively strips away the marked resist layer material. After the drying of the substrate, the patterned resist is finally obtained, which, in conclusion, is generally subjected to a thermal step for curing.
At the end, the minimum structure line width on the mask which is actually produced after the production of the resist structure is greater than the limit resolution calculated from (1) for k=0.25, for a number of reasons. Firstly, the resist layer has a finite thickness, so that the imaging blurs slightly; furthermore, the developer acts isotropically, so that the resist is also removed in the lateral direction during the development of the resist layer. The minimum structure line width on the mask which is required for the production of a resist layer structure on a semiconductor substrate therefore depends on many parameters and is determined individually for each patterning process.
The mask, by way of example, comprises an unpatterned quartz glass substrate which is light-transparent (transmissive) even in the UV region and on which a thin opaque layer, usually made of black chromium, is applied. The black chromium layer produces, together with the transparent regions, the mask structure which is imaged onto the resist layer. In this case, the black chromium layer produces the darkened regions on the resist layer, while the light-transparent region produces the exposed regions on the resist. If the resist is positive, then the resist becomes soft in the exposed regions and is removed by the development step. If the resist is negative, then the resist remains insoluble in the exposed regions, so that the non-exposed regions are removed during development. For a dimensionally accurate structure transfer, it is furthermore important to ensure a homogeneous exposure dose over the region to be exposed.
Various effects can contribute to impairing the dimensional fidelity. Firstly, the finite resist contrast γ, which is a measure of the resist removal gradient, causes rounding of originally cornered mask structures. Furthermore, interference effects, diffraction effects and scattered light which arise at structure elements of the mask, the resist layer and/or the prepatterned substrate surface, and imaging errors, such as lack of focus and lens aberrations, can result in the effective exposure dose not being homogeneous in the resist layer regions.
FIG. 1 illustrates the abovementioned difficulties on a conventional lithography mask, which has a radiation-transparent substrate 11 made of glass, for example, and a radiation-opaque layer 12 made of chromium, for example. In this case, the openings 13, 14 in the radiation-opaque layer 12 correspond to the structure which is intended to be transferred to the photoresist layer on the wafer in the corresponding mask step. During an exposure, radiation, for example ultraviolet light, passes through the openings 13, 14 in the radiation-opaque layer 12 and, on account of interference effects, results in the illustrated distribution of the electric field E in the-photoresist layer on the wafer.
On account of diffraction effects, an undesirable exposure occurs in the photoresist layer between the openings 13 and 14, an actually dark region on the mask. Since the exposure intensity is proportional to the square of the field strength, the field strength distribution shown in FIG. 1 results in a corresponding intensity distribution I in the photoresist layer.
In order to avoid these difficulties and in order to improve the structure resolution, so-called “alternating phase masks” are also increasingly being used, therefore, instead of the previously described so-called “binary masks”. In this case, a phase deviation is applied to every second opening 13 in the radiation-opaque layer 12, for example by etching the glass substrate 11, in such a way that a phase difference is obtained between adjacent openings 13, 14. In this case, 180° is generally set as the phase difference. By using this technique it is possible, in the case of highly periodic, grating-like structures, to obtain an increase in the structure resolution by up to a factor of 2 compared with the conventional technique.
FIG. 2 illustrates the resulting situation. On account of the 180° phase difference between adjacent openings 13, 14, destructive interference now occurs between the radiation which passes through the left-hand opening 13 and the radiation which passes through the right-hand opening 14. Therefore, the field distribution E in the photoresist layer now has a zero between the two openings 13, 14, which accordingly also results in a significantly lower intensity I between the two openings 13, 14. The exposure contrast is significantly improved in this way.
Unfortunately, however, this positive effect occurs only for radiation-opaque structures which have an opening with a phase difference on both sides. Since the patterns formed by the openings correspond to the structures which are intended to be imaged or transferred into the photoresist layer, situations can arise, however, wherein openings with only one adjacent opening or fully isolated openings occur. In this case, it can happen that such a half-isolated or fully isolated opening is not imaged completely into the resist layer. Attempts have been made hitherto to ensure a transfer into the photoresist layer by widening the corresponding openings at least under optimum lithographic conditions (optimum focus, nominal exposure). However, the lithographic process window is then so small that the corresponding structures, in the production process, often lead to a failure of the component. Accordingly, this technique is used only in rare cases in practice, which has the consequence that critical structures in the layout must be prohibited, which results, however, in a drastic limitation in the application of alternating phase masks.
A further possibility for increasing the structure resolution compared with conventional binary masks consists in the use of so-called half-tone phase masks (“half-tone phase shift mask”, HTPSM). To that end, instead of a radiation-opaque layer, a layer which is radiation-transparent to a certain percentage (e.g. 3% to 40% radiation transmission) is used, which shifts the phase of the radiation passing through it by a predetermined magnitude, and is applied to the glass substrate. Afterward, this so-called “half-tone layer” is correspondingly patterned to produce openings in the layer which are matched to the pattern to be transferred. If the mask is then irradiated, a phase jump (generally 180°) occurs at the edges of the openings, as a result of which the attainable resolution can be increased.
FIG. 3 illustrates the resulting situation. The radiation which passes through the openings 13 is not shifted in its phase and has a relatively high intensity. The radiation which passes through the half-tone layer 15 is shifted by 180° in its phase and simultaneously reduced in its intensity. On account of the 180° phase difference between the openings 13 and regions of the half-tone layer 15, a destructive interference occurs at the edge of the openings 13, as a result of which the electric field has a zero and the irradiation contrast is significantly improved.
Unfortunately, however, in the case of half-tone phase masks, the resolution and the process window are relatively small in the case of very densely packed structures with circular illumination. The imaging of these structures can be improved by inclined illumination, although auxiliary structures are required for the simultaneous imaging of isolated structures, which auxiliary structures require a high outlay in the drawing, fabrication and inspection of the masks.